The present invention relates to an anti-jamming and anti-spoofing technique for interrogation systems which utilize time delay as a code means, and more particularly, pertains to the utilization of information which measures a jamming environment. Such an interrogation system is the Mark XII system, an air defense identification system employing a long, encrypted interrogation word, consisting of a multi-pulse train, and a single pulse reply calculated from the interrogation. The Mark XII system is typified by the AN/TPX-46 and AN/TPX-50 Interrogators presently fielded by the U.S. Army. The operation of these interrogators is completely described in the United States Army Technical Manuals designated the TM 11-5895-532-34 series and the TM 11-5895-687-35 series respectively. These manuals are unclassified. The reply code in the Mark XII system is its delay from the time of receipt of interrogation. An enemy who attempts to overcome such an identification system usually does so by spoofing, that is, guessing at the appropriate reply word and time delay subsequent to the interrogation transmission, or by jamming, that is pouring out continuous replies at a great rate.
The concept of "range bins" is fundamental to the understanding of the operation of Mark XII interrogators. The range of an interrogator set may be divided into any number of equal range increments known as range bins. Since the interrogator set itself has no radar capability, ranging information is determined by measuring the time between transmitted and received signals. It is assumed the propagation speed of the signal and the retransmission time of the return signal are well known. While the interrogators may be highly mobile units, they are in a fixed position while interrogating, and therefore the spatial correspondence of each range bin to reply time remains constant.
Range zero as used herein is defined as the range of a target at the shortest range of the interrogator. Since ranging is directly related to time as described above, replies from targets at range zero are expected to arrive at the interrogator receiver some predetermined time after transmission of the interrogation code. Replies arriving at the interrogator receiver prior to range zero cannot, by definition, be associated with the interrogation signal of interest. Clearly then, although the description which follows herein utilizes the terminology "range bins prior to range zero," it will be understood that such range bins do not have associated with them a spatial position and are indicative only of the time periods prior to the arrival time of the earliest expected reply signal. Since a jammer continuously emanates jamming signals, these signals will be received over all time and will therefore fill the range bins both before and after range zero. Range zero is the range of a target at the shortest detectable range of the interrogator. Similarly, a spoofer sometimes replies sufficiently early after the interrogation to fall into a range bin prior to range zero. Friendly replies, however, will only appear after range zero. To date, range interrogators do not have anti-jamming capabilities.